1. Field of Invention
The invention relates to an ultrasonic transducer for installation in an instrument housing, wherein the ultrasonic transducer comprises a transducer housing and a housing fixture and wherein the transducer housing can be put under a medium pressure in its installed state on its emitting and/or receiving side.
2. Description of Related Art
Ultrasonic transducers of the above-mentioned type have been known for years and are, for example, used in acoustic mass flowmeters to a wide extent. The ultrasonic transducer transforms electric energy in the oscillation of a membrane that is provided on the emitting and/or receiving side in the transducer housing; in this case, the ultrasonic transducer acts as an ultrasonic emitter. By the same token, it is also possible that a membrane provided on the emitting and/or receiving side is oscillated by external—occurring in the medium—pressure fluctuations and the oscillation is transformed into a respective signal; in this case, the ultrasonic transducer acts as an ultrasonic receiver. In some applications—as, e.g., fill level measurement—such an ultrasonic transducer is used as both an ultrasonic emitter and an ultrasonic receiver, in the field of mass flow measurement, an ultrasonic transducer is commonly used either as an ultrasonic emitter or as an ultrasonic receiver.
In acoustic mass flow measurement, the effect is usually used that the propagation speed of the acoustic signal is superimposed on the rate of feed of the medium in a medium transported in a measuring tube. The measured propagation speed of the acoustic signal compared to the measuring tube is greater than in a recumbent medium when the medium is transported in the direction of the acoustic signal and the speed of the acoustic signal compared to the measuring tube is less than in a recumbent medium when the medium is transported against the direction of emission of the acoustic signal. The run time of the acoustic signal between the acoustic emitter and the acoustic receiver—both are ultrasonic transducers—depends on the rate of feed of the medium compared to the measuring tube and thus, due to the entrainment effect, compared to the acoustic emitter and the acoustic receiver.
It is a problem for measurements based on emitted acoustic or ultrasonic signals, not only in the field of mass flow measurement, that the ultrasonic oscillation created in the ultrasonic transducer are not only transmitted by the emitting and/or receiving side of the transducer housing in the surrounding medium of the ultrasonic transducer, but that the created oscillations are transmitted to the instrument housing via the transducer housing—as the case may be, via the housing fixture, insofar as it differs from the instrument housing. This is not only a problem because, under certain circumstances, a considerable portion of the transmission power is “lost”, rather is a problem because the ultrasonic waves transmitted to the instrument housing by so-called crosstalk can lead to considerable receiving-side interference. This is accounted for in that, for example, it cannot be differentiated on the receiver side if the received ultrasonic signal was received via the medium—wanted signal—or via the instrument housing, wherein the ultrasonic signal transmitted via the instrument housing, then again creates crosstalk in the transducer housing of the receiving ultrasonic transducer.
In particular, in gas applications, in which the medium is comprised of gas, the portion of the oscillation energy transmitted from the ultrasonic transducer into the gaseous medium compared to the total created oscillation energy is very low, so that the problem of crosstalk is particularly aggravating here.
Different methods are known from the prior art for reducing crosstalk in ultrasonic oscillations from actual oscillators in the transducer housing to the transducer housing and further to the instrument housing. Some of these methods are based on the transmission path from the ultrasonic generator—e.g., a piezo element on the emitting and/or receiving side of the transducer housing—to the transfer at the instrument housing being constructively lengthened. Other methods include attempts at acoustically decoupling the ultrasonic source from the rest of the ultrasonic transducer and/or from the instrument housing, for example by creating acoustic transfers with materials, which result in a poor impedance matching and, thus, cause transmission of a lower energy portion. Multiple methods are often combined with one another.
In gas applications, there is the additional problem that the portion of energy directly transmitted into the gaseous medium is strongly dependent on the pressure and, thus, the density of the medium. Pressure fluctuations lead to the ratio of the wanted signal energy to the crosstalk signal energy being highly varied, whereby the analysis of ultrasonic signals based on signal levels or signal level ratios is made more difficult.